I have extensive Level D experience spanning the better part of a decade, have seen this often, and consider it a bug.

Suppose you are cruising at FL350 and there are 3 crossing restrictions on the STAR over the next 3 waypoints:

ABCEF FL240
GHIJK 250/11000A
LMNOP 210/6000A

If you were to reset the MCP altitude window to 6000, the VNAV logic will descend to 6000, slow to 210, and engage altitude hold. However, if you only reset the altitude window to 11000, the same will happen at 250/11000. The third crossing restriction will not be met unless you reset the altitude window for 6000 with sufficient time to meet it.

When cruising in the FLs, you reset your altimeters to 29.92. If you're using real weather, the actual barometer pressure is changing underneath you. If you fly from an area of high pressure to an area of low pressure, you're effectively descend. You're supposed to be cruising at FL350, but all of a sudden you're at FL348. VNAV gets confused, and kicks into altitude hold. I'm not sure why it does this, but the bug can be recreated at cruise by rapidly decreasing the altimeter setting.

It happens pretty randomly. I think mostly when Activesky does a refresh, but I've never cared enough to pin it down. Just click the VNAV button again, and appreciate how advanced the LD763 was when it was released.

Peter Kerpel

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